1. Field of the Invention
The present invention relates to sensors. More specifically, the present invention relates to thermal stabilization of imaging detector arrays.
2. Description of the Related Art
This invention addresses a problem that can occur with heat sensitive detector arrays used for infrared imaging systems. On the displayed video, a field-to-field flicker can occur when the detector array is sampled at a rate that is synchronous with interlaced display formats such as NTSC or PAL. For interlaced formats, the timing is asymmetrical for the odd and even fields. That is, the time duration from the odd field to its spatial equivalent point on the even field is different than that from the even field to the odd field. To avoid design complexity, most imaging systems apply the interlaced display timing format to the detector assembly directly.
For highly heat sensitive detectors such as bolometers and other uncooled detectors, the output signal from each pixel is a function of that pixel's temperature. Ideally, the pixel's temperature would only change as the image that is projected onto the array changes. However, the process of sampling or sensing the array information increases the temperature of the array. During the time the array is not being sampled, the array cools. Sampling at uneven intervals, as would be consistent with interlaced timing, causes a pixel to cool to a slightly different temperature every other field. This causes unwanted offsets between fields, with a magnitude that can vary nonlinearly as a function of operating time and absolute temperature. The result of the offset is a field-to-field video picture bounce or flicker, which is unpleasant to the human eye.
Prior attempts to solve this problem involved the averaging of two consecutive fields. For detector arrays where the detector size is equal to the display field size (i.e., the same detector rows are used for both fields), the displayed field was created by averaging two consecutive fields read from the detector. This eliminates the flicker but reduces the sensitivity or resolution of the system and causes blurring during scene motion. For detector arrays that have a unique row set for each display field (for example, the odd rows displayed on one field and the even rows displayed on the other field), averaging would not work unless the entire array was sampled every field, even though only every other line is being displayed. This would require doubling the sampling frequency which is usually not a feasible option. Simple methods of adding a fixed offset to a given field could reduce the problem, but since the offset can drift over time and temperature it would not be eliminated.
Hence, a need exists in the art for an improved system or method for sampling and displaying output from detector arrays that avoids the problems associated with asymmetrical display timing.